1. Field of the Invention
The present invention relates to a microphone circuit that includes an external power supply circuit, and obtains a microphone output even in a case where an external power is supplied or in a case where the external power is not supplied, by automatically switching a circuit corresponding to the respective cases.
2. Related Background of the Invention
Microphones can be grouped into a dynamic microphone, a ribbon type microphone, a condenser microphone and the like according to the transducing form of a microphone unit as an electroacoustic transducing module. The dynamic microphone and the ribbon type microphone have the advantage of being capable of carrying out electroacoustic conversion without a power supply, but they have the drawback that the output level is low. Accordingly, the dynamic microphone or the ribbon type microphone has a problem in that if an output cable is extended, noise penetrates into an output cable, which is a transmission line of a sound signal, due to the low output level, thereby causing a significant degradation of S/N ratio of the output signal.
In order to solve such problem, an amplifier is incorporated in the microphone side to amplify a signal at the microphone side and thus increase the signal level for transmission. In order to operate the above-described amplifier at the microphone side, a power supply is required, and thus as a means for supplying power to the microphone side, an amplifier module that operates with a phantom power supply is commercialized, paying attention to the phantom power supply that is often used in a condenser microphone. When the microphone output needs to be amplified at the microphone side, the above-described amplifier module is connected between the microphone and the microphone cable and the phantom power supply is connected, thereby supplying a phantom power from the outside. By doing this way, the above-described amplifier module operates to amplify the microphone output and thus increase the output level, so that even if noise penetrates when transmitting the microphone output via the microphone cable, the degradation of S/N ratio can be reduced.
The above-described phantom power is supplied to the microphone via the output cable of the microphone. The microphone cable in this case is typically constructed as to be removably mounted to the microphone by a 3-pin microphone connector defined by EIAJ RC-5236 “a latch-lock round connector for an audio system.” In the above-described 3-pin microphone connector, typically, a first pin is used for grounding, a second pin used as the hot side of a signal, and a third pin used as the cold side of the signal. A ground line of the microphone cable, more specifically, a shielded line is connected to the first pin. A signal line at the hot side of the microphone cable is connected to the second pin, and a signal line at the cold side is connected to the third pin.
If the amplifier module is mounted between a dynamic microphone and the microphone cable, the dynamic microphone is physically coupled to the amplifier module, thus causing a problem that the microphone portion is enlarged. The above-described problem can be dissolved by interposing a cable between the dynamic microphone and the amplifier module. However, interposing of the above-described cable increases the number of connection points by a connector, thus increasing the factors to cause contact failure and the factors to cause noise penetration due to the presence of the connecting portion by to the connector.
A product made by incorporating the amplifier, which operates with a phantom power supply, into a ribbon type microphone is also commercialized. This ribbon type microphone with the built-in amplifier is however a product on condition that there is a phantom power supply, and thus without the phantom power supply it is impossible to obtain the output and thus this ribbon type microphone can not be used.
In addition, as a prior art related to the present invention, there is proposed a microphone power supply device including: a gain detector for detecting a gain of a microphone amplifier to which a microphone is connected; a gain determination unit that determines an apparatus, which is connected to the microphone amplifier, based on the gain which the gain detector detects; a power supply controller for controlling power supply to the microphone; and a power supply unit for supplying power to the microphone, wherein the power supply controller is controlled based on the gain of the microphone amplifier (e.g., see Patent Document 1: Japanese Patent Application Laid-open No. 2003-116193).
However, the invention described in Patent Document 1 is the one for automatically controlling the power supply controller based on the gain of the apparatus connected to the microphone amplifier, and as long as the automatically controlling of the power supply controller is concerned, the invention described in Patent Document 1 is related to the present invention, however, it differs from the present invention in the problem to be solved and in the means for solving the problem.